The present invention relates to a folding electronic device and, more particularly, to a folding electronic device including left- and right-channel speakers which can be driven independently.
For the purpose of improving portability, most types of portable electronic devices (to be also simply referred to as electronic devices hereinafter) such as cellular phones are configured such that a display unit which displays character information or image information and an operation unit on which keys for information input are arranged are integrated into a foldable unit. As disclosed in Japanese Patent Laid-Open No. 2001-177609 (reference 1), some current folding cellular phones have a music function as well as a telephone function.
Of cellular phones with a music function, popular cellular phones are those which include left- and right-channel speakers which can be driven independently and are designed to improve acoustic quality by outputting stereo sounds. A folding cellular phone having such an arrangement allows the user to enjoy stereo sounds while watching images on the display unit by driving the left- and right-channel speakers even in the closed state of the cellular phone in which it is folded as well as in the open state in which the cellular phone is not folded.
As shown in FIG. 7, a conventional folding cellular phone 50 having left- and right-channel speakers which can be driven independently includes a radio transmission/reception unit 52 which transmits/receives radio signals through an extendible transmission/reception antenna 51, a control unit 53 which forms the main part of the cellular phone 50, a memory unit 55 which stores external sound source data downloaded through the radio transmission/reception unit 52, internal sound source data, and the like as sound source data 54, a detection switch 56 which detects the open or closed state of the cellular phone 50, and a left (L)-channel speaker 57L and right (R)-channel speaker 57R which can be driven independently.
The control unit 53 is comprised of a switching control unit 58 formed of a CPU (Central Processing Unit) which controls the switching operation of a switch (to be described later) in accordance with the detection result obtained by the detection switch 56, a left-channel DA (Digital to Analog) converter 59L and right-channel DA converter 59R which convert the sound source data 54 stored in the memory unit 55 into analog data, and a left-channel amplifier 60L and right-channel amplifier 60R which respectively amplify outputs from the DA converters 59L and 59R. Reference numeral 61 denotes a pull-up resistor connected to a power supply; and 62L and 62R, capacitors.
FIG. 8 shows another conventional cellular phone having left- and right-channel amplifiers formed of BTL (Balanced Transformerless Amplifier) amplifiers. A folding cellular phone 70 includes a left-channel BTL amplifier 72L and right-channel BTL amplifier 72R. The BTL amplifier 72L is comprised of first and second amplifiers 71A and 71B which amplify an output from the left-channel DA converter 59L. The BTL amplifier 72R is comprised of third and fourth amplifiers 74A and 74B which amplify an output from the right-channel DA converter 59R. Phase inversion circuits 75L and 75R are respectively connected to the input terminals of the amplifier 71B of the BTL amplifier 72L and the amplifier 74B of the BTL amplifier 72R. Except for this, the arrangement of this cellular phone is almost the same as that shown in FIG. 7. Therefore, the same reference numerals as in FIG. 7 denote the same components in FIG. 8, and a description thereof will be omitted.
In the conventional folding cellular phones 50 and 70 shown in FIGS. 7 and 8, sounds output from the speakers 57L and 57R flow in laterally opposite directions in the open state and closed state. This results in unnatural sound reproduction in terms of human hearing sense. More specifically, in the open state, stereo sounds are output from the speakers 57L and 57R in synchronism with images on the display unit, and hence natural sounds can be produced in terms of human hearing sense like sounds produced when a car passes by. In the closed state, however, since the positions of the speakers 57L and 57R become laterally opposite to those in the open state, output stereo sounds are not synchronized with images. For this reason, the sounds inevitably become unnatural in terms of human hearing sense.
An audio set designed to improve such unnaturalness in terms of human hearing sense is disclosed in, for example, Japanese Patent Laid-Open No. 2002-78073 (reference 2). As shown in FIG. 9A, in the audio set disclosed in reference 2, a main body 100 includes left and right speaker boxes 103 and 102 respectively housing speakers 107 and 106, which are provided on side surfaces of the main body so as to be pivotal in the back-and-forth direction as indicated by the arrows. This audio set is provided with an angle detection means (not shown) for detecting a state in which the boxes 103 and 102 pivot backward through 90° or more as shown in FIG. 9B. In such an arrangement, when the angle detection means detects that both the left and right speaker boxes 103 and 102 have pivoted backward through 90° or more, drive signals to the speakers 107 and 106 are interchanged.
In the audio set disclosed in reference 2, since control is made to interchange drive signals to the speakers 107 and 106 when the angle detection means mechanically detects that the speakers 107 and 106 have pivoted through a predetermined angle or more, the control becomes complicated. In addition, since this set requires the speaker boxes 103 and 102 housing the speakers 107 and 106, it is difficult to apply this technique to folding electronic devices like folding cellular phones which are required to satisfy conditions for excellent portability in terms of size, weight, and the like.
These problems will be described in detail. In the audio set disclosed in reference 2, the speaker boxes 103 and 102 are configured to pivot backward through 90° or more from the open state to the closed state. For this reason, only when the speaker boxes 103 and 102 have pivoted through 90° or more, drive signals to the speakers 107 and 106 must be interchanged. This complicates the control.
The audio set disclosed in reference 2 is directed to an electronic device which is not required to have portability in terms of size, weight, and the like and allowed to include the speaker boxes 103 and 102 housing the speakers 107 and 106. It is difficult for an electronic device required to satisfy conditions for excellent portability in terms of size, weight, and the like to include the speaker boxes 103 and 102 housing the speakers 107 and 106. Even if a device includes the speaker boxes 103 and 102, it is practically impossible to pivot the speaker boxes 103 and 102 to the back side of the main body.